In the recent development of semiconductor fabrication technology, the continuous miniaturization in device fabricated demands more stringent requirements in the fabrication environment and contamination control. When the feature size was in the 2 .mu.m range, a cleanliness class of 100.about.1000 (which means the number of particles at sizes larger than 0.5 .mu.m per cubic foot) was sufficient. However, when the feature size is reduced to 0.25 .mu.m, a cleanliness class of 0.1 is required. It has been recognized that an inert minienvironment may be the only solution to future fabrication technologies when the device size is reduced further. In order to eliminate micro-contamination and to reduce native oxide growth on silicon surfaces, the wafer processing and the loading/unloading procedures of a process tool must be enclosed in an extremely high cleanliness minienvironment that is constantly flushed with ultrapure nitrogen that contains no oxygen and moisture.
Different approaches in modern clean room design have been pursued in recent years with the advent in the ULSI technology. One is the utilization of a tunnel concept in which a corridor separates the process area from the service area in order to achieve a higher level of air cleanliness. Under the concept, the majority of equipment maintenance functions are conducted in low-classified service areas, while the wafers are handled and processed in more costly high-classified process tunnels. For instance, in a process for 16 M and 64 M DRAM products, the requirement of contamination control in a process environment is so stringent that the control of the enclosure of the process environment for each process tool must be considered. This stringent requirement creates a new minienvironment concept which is shown in FIG. 1. Within the enclosure of the minienvironment of a process tool 10, an extremely high cleanliness class of 0.1 (which means the number of particles at sizes larger than 0.1 .mu.m per cubic foot) is maintained, in contrast to a cleanliness class of 1000 for the overall production clean room area 12. In order to maintain the high cleanliness class inside the process tool 10, the loading and unloading sections 14 of the process tool must be handled automatically by an input/output device such as a SMIF (standard mechanical interfaces) apparatus. A cassette of wafers can be transported into the process tool 10 by a SMIF pod 18 situated on top of the SMIF apparatus 20.
In a conventional SMIF apparatus 20 such as that shown in FIG. 1, the apparatus 20 consists of a robotic transfer system (not shown) or a robotic arm which is normally configured for gripping the top of a cassette 30 from a platform on which the cassette 30 placed (inside a pod). The robotic arm, sometimes replaced by a gripper assembly, is capable of transporting the cassette 30 into the process tool and place it onto a platform 16 vertically such that the cassette 30 is oriented horizontally. At the beginning of the process, an operator positions a SMIF pod 18 on top of a platform/elevator 22 which contains a cassette 30 holding, for instance, 24 wafers in an upright position. The elevator then descends into the SMIF apparatus 20 for the robotic arm to transport the cassette 30 into the process tool. The SMIF system 20 is therefore capable of automatically utilizing clean isolation technology to maintain a high class clean room effectiveness near wafers and processing equipment. The operation of the robotic arm or the gripper is controlled by an ancillary computer (not shown) or by the process tool 10. The cassette 30 carries wafers or other substrates that are being processed.
The SMIF apparatus 20 has a port (or opening) 24 which is intimately mated with an opening 26 in the sidewall 28 of the process tool 10. The SMIF pod 18 is a sealed container with an opening at the bottom and therefore is capable of preventing contamination to the cassette held therein. The pod may also be equipped by a tagging system for the automated identification and recognition of the parts contained in the pod to prevent mis-processing of the wafers and to track through the host computer of the product-lot serial numbers. The tagging system is mounted on the pod with a probe assembly mounted on the port of the SMIF apparatus 20. The SMIF apparatus 20 is therefore an effective interface between an operator and the process tool 10 in that the transporting of cassette can be conducted in a completely automated fashion to avoid human contact by the operator. This insures that the cassette 30 is transported through a highly clean environment into the process tool 10.
A cross-sectional view of the SMIF pod 18 is shown in FIG. 2. It is seen that the wafer cassette 30 is contained in the pod 18 by inserting the cassette 30 through an open bottom 32 of the pod 18. The cassette 30 is used to store a plurality of wafers 34 inside the wafer cassette 30, a plurality of dividers (not shown) are provided on the two interior sidewalls (not shown) of the cassette body 36 such that wafer receptacles are formed between the dividers. A bottom wall 38 of the wafer cassette 30 also performs the functions as a seal for the bottom opening 32 of the SMIF pod 18.
In the manufacturing process for semiconductor devices, wafers that are contained in a wafer cassette are frequently transported in a SMIF pod 18. The SMIF pod is broadly used not only in transporting wafer cassettes between various processing stations, but also in storing wafers in cassettes for those wafers waiting to be processed in a process machine. During such storage, the content of the atmosphere inside the SMIF pod 18 is the same as the clean room atmosphere. The clean room atmosphere contains a regulated amount of moisture, i.e., between about 30% and about 50% relative humidity to avoid excessive static charge build-up and to afford personnel comfort. The same moisture content exists inside the SMIF pod 18 since the pod is positioned in such atmosphere. The moisture content has adverse effect on the wafers stored in the wafer cassette placed in the SMIF pod. The effect is especially severe when the wafers have been deposited, or otherwise formed, with an oxide layer on top of the wafer surface. The moisture readily reacts with the oxide film and causes film stress variations and even corrosion underlayer of metal film. The film stress variations on the surface of a wafer induce other problems in the subsequent deposition or forming processes for IC devices.
The moisture absorption problem incurred in the conventional SMIF pod has lead IC process engineers to come up with various solutions. For instance, in some processes the queue time is set as short as possible such that the exposure of wafer to moisture is minimized. However, depending on the process sequence, a preset short queue time between processes is not always possible. Another solution to the moisture absorption problem incurred in the SMIF pod was proposed in which the SMIF pod is placed in a nitrogen box such that the pod is isolated from the clean room air. Even though this may be an effective process, the preparation and positioning of a large number of nitrogen boxes which are connected to a nitrogen gas supply is extremely difficult and cumbersome to carry out in an IC device fabrication facility.
It is therefore an object of the present invention to provide a method and apparatus for storing wafers that do not have the drawbacks and shortcomings of the conventional wafer storage method and apparatus.
It is another object of the present invention to provide a method for storing wafers without the moisture absorption problem by utilizing a SMIF pod and a wafer cassette therein for holding a plurality of wafers and an inert gas therein.
It is a further object of the present invention to provide a method for storing wafers without the moisture absorption problem which does not require a preset short queue time.
It is another further object of the present invention to provide a method for storing wafers without the moisture absorption problem that does not require the use of a nitrogen box for placing a SMIF pod therein.
It is still another object of the present invention to provide a method for storing wafers without the moisture absorption problem by modifying a SMIF pod such that a continuous flow of nitrogen is fed into the pod.
It is yet another object of the present invention to provide a method for storing wafers without the moisture absorption problem by feeding a continuous flow of nitrogen into a hollow handle of a SMIF pod.
It is still another further object of the present invention to provide a method for storing wafers without the moisture absorption problem by flowing a continuous flow of nitrogen into a SMIF pod such that a positive pressure exists between the pod interior and the surrounding environment.
It is yet another further object of the present invention to provide an apparatus for storing wafers without the moisture absorption problem by supplying a container that is equipped with a gas supply means in fluid communication with a cavity of the container such that a constant flow of an inert gas can be fed into the cavity,